We have developed and characterized the only myelogenous leukemia cell line (K-562) available originally derived from a patient with CML. The K-562 cell line has the Philadelphia (Ph1 plus) chromosome and Fc receptor after nine years in culture. The cell line has been shown to be pluripotential under appropriate stimulation that differentiates into recognizable progenitors of the erythrocytic, granulocytic, monocytic and megakaryocytic series. These K-562 cells, when injected into rabbit, monkey or goat, elicit the production of specific antibody which demonstrates complement mediated cytotoxicity for the leukocytes of patients with leukemia. Specificity for the homologous cell line is obtained after absorption of antisera with normal peripheral leukocytes and bone marrow cells as well as with leukemia cells of other types (e.g., AML, ALL, CLL). A single polypeptide (77,000 - 80,000 daltons) has been isolated from the K-562 cells that specifically inhibits the cytotoxicity assay. The antibody can cause the cytolysis of the cells in the absence of complement or effector cells. The purpose of this project is: 1) to further characterize the K-562 cell antigen biologically and biochemically; 2) to assess the presence of antibody, antigen, or complexes in the sera of patients with leukemia using the isolated antigens and monospecific antisera; 3) to use the immunologic reagents (monospecific serum and characterized antigens) as reference standards to accurately detect incomplete remission and early relapses in patients with leukemia.